JP7684393B2 - Separator for lithium secondary battery and method for producing same - Google Patents

Description

The present invention relates to a separator that can be used in electrochemical devices such as lithium secondary batteries and a method for manufacturing the same.

This application claims priority to Korean Patent Application No. 10-2020-0125066, filed on September 25, 2020, the entire contents of which are incorporated herein by reference in their entirety in the specification and drawings.

In recent years, interest in energy storage technology has been growing. As the range of applications expands to include mobile phones, camcorders, and laptops, as well as the energy of electric vehicles, efforts in the research and development of electrochemical elements have become more and more concrete. From this perspective, electrochemical elements are the field that has attracted the most attention, and in particular, the development of rechargeable secondary batteries has attracted attention. In recent years, research and development has been conducted on the design of new electrodes and batteries to improve the capacity density and specific energy of such batteries.

Among the secondary batteries currently in use, lithium secondary batteries, developed in the early 1990s, have been attracting attention due to their advantages of high operating voltage and significantly higher energy density compared to conventional batteries that use aqueous electrolytes, such as Ni-MH, Ni-Cd, and sulfuric acid-lead batteries.

Electrochemical devices such as lithium secondary batteries are produced by many manufacturers, but their safety characteristics vary from one another. It is very important to evaluate and ensure the safety of such electrochemical devices. For example, the separator prevents short circuits between the positive and negative electrodes and at the same time provides a path for lithium ions to move, making it an important factor that affects the safety and output characteristics of the battery.

Separators for lithium secondary batteries are hydrophobic because their main raw material is stretched polyolefin. Therefore, the thicker the porous polymer substrate, the less easily it can be impregnated with electrolyte.

The problem that the present invention aims to solve is to provide a separation membrane in which the properties of a porous polymer substrate are modified from hydrophobic to hydrophilic.

This also aims to improve the performance of batteries that use such separators.

One aspect of the present invention provides a separator for a lithium secondary battery according to the following embodiment.

According to the first embodiment,
Provided is a separator for a lithium secondary battery, comprising: a porous polymer substrate; and a hydrophilic modified layer located inside the porous polymer substrate or on at least one surface of the porous polymer substrate, the hydrophilic modified layer including a polymer compound represented by the following Formula 1 and a surfactant:

In chemical formula 1, n is an integer between 6 and 8.

According to the second embodiment, in the first embodiment,
The polymeric compound may include α-cyclodextrin, β-cyclodextrin, γ-cyclodextrin, or two or more of these.

According to a third embodiment, in the first or second embodiment,
The content of the polymer compound may be 1 part by weight or less based on 100 parts by weight of the porous polymer substrate.

According to a fourth embodiment, in any one of the first to third embodiments,
The surfactant may be a fluorosurfactant, a silicone surfactant, a nonionic surfactant, a cationic surfactant, an anionic surfactant, or a mixture of two or more of these.

According to a fifth embodiment, in any one of the first to fourth embodiments,
The surfactant has an electrolyte permeability of 30% or more even after storage at high temperature, and the electrolyte permeability can be measured using an ultraviolet-visible spectrophotometer at room temperature (25° C.) after a step of preparing a composition by adding 0.5 parts by weight of the surfactant to 100 parts by weight of an electrolyte containing 1 M _LiFP6 in ethylene carbonate and ethyl methyl carbonate (3:7 volume ratio), sealing the prepared composition, and storing it in an oven at 70° C. for 6 hours.

According to a sixth embodiment, in any one of the first to fifth embodiments,
The amount of the surfactant may be 0.5 parts by weight or less based on 100 parts by weight of the porous polymer substrate.

According to the seventh embodiment, in any one of the first to sixth embodiments,
At least one surface of the separator may include a porous coating layer including inorganic particles and a binder polymer that fixes and connects the inorganic particles.

According to the eighth embodiment, in the seventh embodiment,
The porous coating layer may further include a polymer compound represented by the following Formula 1:

In chemical formula 1, n is an integer between 6 and 8.

According to the ninth embodiment, in the eighth embodiment,
The content of the polymer compound represented by Formula 1 may be 10 parts by weight or less based on 100 parts by weight of the inorganic particles.

According to a tenth embodiment, in any one of the seventh to ninth embodiments,
The binder polymer may be particulate or non-particulate.

According to an eleventh embodiment, in any one of the seventh to tenth embodiments,
The inorganic particles may be inorganic particles having a dielectric constant of 5 or more, inorganic particles having lithium ion transport ability, or a mixture of two or more thereof.

According to the twelfth embodiment, in any one of the seventh to eleventh embodiments,
The inorganic particles may be hydrophilic.

According to the thirteenth embodiment, in any one of the seventh to twelfth embodiments,
The inorganic particles may have a silane group grafted onto the surface thereof.

Another aspect of the present invention provides a lithium secondary battery according to the following embodiment:

According to a fourteenth embodiment,
The battery includes a positive electrode, a negative electrode, and a separator and an electrolyte interposed between the positive electrode and the negative electrode,
The separator may be a separator according to any one of the first to thirteenth embodiments.

Another aspect of the present invention provides a method for manufacturing a separator for a lithium secondary battery according to the following embodiment.

According to the fifteenth embodiment,
(S1) preparing a composition for modifying a substrate in which a polymer compound represented by the following formula 1 and a surfactant are dispersed or dissolved in an aqueous solvent;
(S2) applying the composition for modifying the substrate onto at least one surface of a porous polymer substrate having a large number of pores and being hydrophobic, and drying the composition.

In chemical formula 1, n is an integer between 6 and 8.

According to the sixteenth embodiment, in the fifteenth embodiment,
The step (S2) may be a step of modifying the porous polymer substrate to be hydrophilic by permeating the interior of the porous polymer substrate with the surfactant and the polymer compound.

According to the seventeenth embodiment, in the fifteenth or sixteenth embodiment,
After step (S2),
The method may further include a step (S3) of drying and coating a composition for forming a porous coating layer, the composition including a solvent, inorganic particles, and a binder polymer, on at least one surface of the porous polymer substrate on which the composition for modifying a substrate has been coated and dried to form a porous coating layer.

According to the eighteenth embodiment, in the seventeenth embodiment,
The solvent may be an aqueous solvent or an organic solvent.

According to the 19th embodiment, in the 17th or 18th embodiment,
The composition for forming a porous coating layer may further include a polymer compound represented by Formula 1.

According to the twentieth embodiment, in any one of the seventeenth to nineteenth embodiments,
The step (S3) may be carried out at a relative humidity of 30 to 80%.

According to one embodiment of the present invention, the impregnation of the electrolyte and the migration speed of lithium ions in the separator can be increased by using a polymer compound and a surfactant with a unique structure.

As a result, batteries using such a separator can have improved output and cycle characteristics.

It is also possible to provide a method for manufacturing the separator and the battery.

The drawings attached to this specification are intended to illustrate preferred embodiments of the present invention and serve to further understand the technical ideas of the present invention as well as the contents of the invention, and therefore the present invention should not be interpreted as being limited to only the matters depicted in the drawings. Meanwhile, the shape, size, scale, or ratio of elements in the drawings attached to this specification may be exaggerated to emphasize a clearer description.

4 is a photograph showing the results of an electrolyte impregnation test for examining the electrolyte impregnation correlation of the separator according to Example 1. 4 is a photograph showing the results of an electrolyte impregnation test for examining the electrolyte impregnation correlation of the separator according to Comparative Example 1. 11 is a photograph showing the results of an electrolyte impregnation test for examining the electrolyte impregnation correlation of the separator according to Comparative Example 2. 1A and 1B are photographs showing the separator according to Example 1 immediately after a heat shrinkage test. 6A and 6B are photographs showing the separator according to Comparative Example 1 immediately after a heat shrinkage test.

Below, the embodiment of the present invention will be described in detail. Prior to this, the terms and words used in this specification and claims should not be interpreted as being limited to their ordinary and dictionary meanings, but should be interpreted as having meanings and concepts that correspond to the technical ideas of the present invention, in accordance with the principle that the inventor himself can appropriately define the concepts of terms in order to best describe the invention. Therefore, it should be understood that the examples described in this specification and the configurations shown in the drawings are merely the most preferred embodiment of the present invention, and do not represent the entire technical ideas of the present invention, and therefore there may be various equivalents and modifications that can be substituted for them at the time of this application.

Throughout this specification, when a part "includes" or "has" other components, it means that it may further include the other components, not excluding the other components, unless otherwise specified.

In addition, the terms "about," "substantially," and the like, used throughout this specification, when given manufacturing and material tolerances inherent in the stated meaning, are used to mean a numerical value or close to that numerical value, and are used to prevent unscrupulous infringers from unfairly taking advantage of disclosures in which precise or absolute numerical values are stated to aid in the understanding of this application.

One aspect of the present invention relates to a separator for a lithium secondary battery and a method for producing the same.

The separator used in electrochemical elements prevents short circuits between the positive and negative electrodes while providing a path for lithium ions to move, making it an important factor affecting the safety and output characteristics of the battery.

Separators for lithium secondary batteries are hydrophobic because their main raw material is a polymer such as stretched polyolefin. Therefore, the thicker the porous polymer substrate, the less easily it can be impregnated with electrolyte.

In order to facilitate the impregnation of the electrolyte, inorganic particles were introduced onto the porous polymer substrate, but even in this case, the impregnation of the electrolyte was not significantly improved. Even when the surface of the porous polymer substrate was modified by corona surface treatment, there was a problem that the surface modification was not sustained. Other methods of introducing hydrophilic organic additives onto the porous polymer substrate were not successful. Finally, an attempt was made to modify the interface properties of the porous polymer substrate from hydrophobic to hydrophilic using a dispersant, but the effect was insufficient.

One aspect of the present invention is intended to solve the above problems. In other words, the present invention aims to dramatically improve the performance (output and capacity retention rate) of the battery by allowing the electrolyte to smoothly penetrate into the pores in the separator.

According to one aspect of the present invention, a separator for a lithium secondary battery is
The present invention includes a porous polymer substrate and a hydrophilic modified layer located inside the porous polymer substrate or inside and on at least one surface of the porous polymer substrate, the hydrophilic modified layer including a polymer compound represented by the following Chemical Formula 1 and a surfactant:

In chemical formula 1, n is an integer between 6 and 8.

As described above, the separation membrane according to one aspect of the present invention necessarily includes a porous polymer substrate containing a large number of fibrils, a polymer compound represented by Chemical Formula 1, and a surfactant. In this case, the polymer compound has a unique structure, and when included in the separation membrane together with the surfactant, it facilitates the penetration of the electrolyte into the inside of the separation membrane. This is because the surfactant allows the hydrophilic polymer compound to approach closely to the surface of the fibrils in the hydrophobic porous polymer substrate. As a result, the electrolyte can be captured inside the hollow portion of the polymer compound represented by Chemical Formula 1. As a result, the hydrophobic porous polymer substrate can be modified to be hydrophilic.

The separation membrane according to one embodiment of the present invention contains a polymer compound represented by the following chemical formula 1 within a porous polymer substrate.

In chemical formula 1, n is an integer between 6 and 8.

The polymeric compound may be a cyclodextrin in which 6 to 8 glucose units are arranged in a ring. In this case, when the number of glucose units is 6, it is called α-cyclodextrin, when it is 7, it is called β-cyclodextrin, and when it is 8, it is called γ-cyclodextrin. Therefore, in the present invention, the polymeric compound may be α-cyclodextrin, β-cyclodextrin, γ-cyclodextrin, or two or more of these.

In a specific embodiment of the present invention, the content of the polymer compound may be 1 part by weight or less, 0.8 parts by weight or less, or 0.5 parts by weight or less, and 0.01 parts by weight or more, or 0.3 parts by weight or more, based on 100 parts by weight of the porous polymer substrate. The amount of the polymer compound within the above numerical range is appropriate, and the hydrophilicity of the porous polymer substrate can be increased to achieve the effects of the present invention.

The separation membrane according to one aspect of the present invention contains a surfactant within a porous polymer substrate.

As described above, the surfactant is an auxiliary that helps the polymeric compound approach the fibrils in the porous polymeric substrate.

In a specific embodiment of the present invention, the surfactant may be a fluorosurfactant, a silicone surfactant, a nonionic surfactant, a cationic surfactant, an anionic surfactant, or a mixture of two or more of these.

Among these, it is particularly desirable to use fluorosurfactants. Fluorosurfactants have the advantage that they have better coating properties for porous polymer substrates than nonionic surfactants, and have a high oxidation potential, reducing the risk of side reactions in the battery.

In a specific embodiment of the present invention, the fluorosurfactant may contain 5 to 25 fluorines per molecule and may be one or more compounds selected from those represented by the following structural formula:

In the above structural formula, n and m are each an integer from 1 to 10.

An example of the fluorosurfactant is 2-[methyl[(nonafluorobutyl)sulfonyl]amino]ethyl acrylate (FC4430, manufactured by 3M).

In a specific embodiment of the present invention, the surfactant may have an electrolyte permeability of 30% or more, or 40 to 90%, even after storage at high temperature. In this case, the electrolyte permeability may be measured using an ultraviolet-visible spectrometer at room temperature (25° C.) after a step of preparing a composition by adding _0.5 parts by weight of the surfactant to 100 parts by weight of an electrolyte solution containing 1 M LiFP6 in ethylene carbonate and ethyl methyl carbonate (3:7 volume ratio), sealing the prepared composition, and storing it in an oven at 70° C. for 6 hours.

If the electrolyte permeability of the surfactant satisfies this range, it is advantageous in terms of controlling side reactions in the battery.

In a specific embodiment of the present invention, the content of the surfactant may be 1.0 parts by weight or less, 0.7 parts by weight or less, or 0.5 parts by weight or less, and 0.05 parts by weight or more, 0.1 parts by weight or more, or 0.5 parts by weight or more, based on 100 parts by weight of the polymer compound. The amount of the polymer compound within the above numerical range is appropriate, and the hydrophilicity of the porous polymer substrate can be increased to achieve the effects of the present invention. In addition, within the above range, side effects on battery performance can be minimized.

Meanwhile, the separation membrane according to one aspect of the present invention may further include a porous coating layer comprising inorganic particles and a binder polymer on at least one surface of the separation membrane.

In this case, the inorganic particles are not particularly limited as long as they are electrochemically stable. That is, the inorganic particles that can be used in the present invention are not particularly limited as long as they do not cause oxidation and/or reduction reactions within the operating voltage range of the applied electrochemical device (e.g., 0 to 5 V based on Li/Li ⁺ ). In particular, when inorganic particles having a high dielectric constant are used as the inorganic particles, they can contribute to increasing the degree of dissociation of an electrolyte salt, for example, a lithium salt, in the liquid electrolyte, thereby improving the ionic conductivity of the electrolyte solution.

For the reasons mentioned above, the inorganic particles may be inorganic particles having a dielectric constant of 5 or more, inorganic particles having lithium ion transfer capability, or a mixture of two or more of these.

The inorganic particles having a dielectric constant of 5 or more include Al ₂ O ₃ , SiO ₂ , ZrO ₂ , AlO(OH), Al(OH) ₃ , TiO ₂ , BaTiO ₃ , Pb(Zr _x Ti _1-x )O ₃ (PZT, where 0<x<1), Pb _1-x La _x Zr _1-y Ti _y O ₃ (PLZT, where 0<x<1, 0<y<1), (1-x)Pb(Mg _1/3 Nb _2/3 )O ₃ -xPbTiO ₃ (PMN-PT, where 0<x<1), hafnia (HfO ₂ ), SrTiO ₃ , SnO ₂ , CeO ₂ , MgO, NiO, CaO, ZnO and SiC, or a mixture of two or more selected from the group consisting of these.

The inorganic particles having lithium ion transfer ability include lithium phosphate ( _Li3PO4 ), lithium _titanium phosphate ( _LixTiy ( _PO4 ) ₃ , 0<x<2, ₀ <y<3), lithium aluminum titanium phosphate ( _LixAlyTiz ( _PO4 ) ₃ , 0<x<2, 0<y<1, 0<z<3), (LiAlTiP) _xOy - _based glass (0<x<4, 0<y _< 13), _{lithium lanthanum titanate} ( _LixLayTiO3 , 0<x<2, 0<y<3), lithium _{germanium thiophosphate} ( _{LixGeyPzSw ,} 0<x<4, 0<y<1, 0<z<1, 0<w<5), lithium nitride ( _{LixN y} , 0<x<4, 0<y<2), SiS2 _- based glass (Li _x Si _y S _z , 0<x<3, 0<y<2, 0<z<4), and _{P2S5 -} based glass (Li _x P _y S _z , 0<x<3, 0<y<3, 0<z<7).

The average particle size of the inorganic particles is not particularly limited, but is preferably in the range of 0.001 to 10 μm, more preferably 100 nm to 2 μm, and even more preferably 150 nm to 1 μm, in order to form a porous coating layer of uniform thickness and appropriate porosity.

In a specific embodiment of the present invention, the inorganic particles may be hydrophilic. The use of hydrophilic inorganic particles can further increase the electrolyte impregnation. For example, the inorganic particles may be Al(OH) ₃ or Mg(OH) ₂ .

In a specific embodiment of the present invention, the porous coating layer may include a binder polymer having a glass transition temperature (T _g ) of −200° C. to 200° C., which is because it can improve the mechanical properties, such as flexibility and elasticity, of the finally formed porous separator. Such a binder polymer acts as a binder that connects and stably fixes inorganic particles to each other, thereby contributing to preventing deterioration of the mechanical properties of the porous separator.

The binder polymer does not necessarily have to have ion-conducting ability, but when a polymer having ion-conducting ability is used, the performance of the electrochemical element can be further improved. Therefore, the binder polymer can be one having as high a dielectric constant as possible. In fact, since the degree of dissociation of salt in the electrolyte depends on the dielectric constant of the electrolyte solvent, the higher the dielectric constant of the binder polymer, the more the degree of dissociation of salt in the electrolyte can be improved. The dielectric constant of such a binder polymer can be in the range of 1.0 to 100 (measurement frequency = 1 kHz), and can be particularly 10 or more.

In addition to the above-mentioned functions, the binder polymer may have a feature of exhibiting a high degree of swelling with the electrolyte by gelling when impregnated with the liquid electrolyte. Therefore, the solubility index, i.e., Hildebrand solubility parameter, of the binder polymer is in the range of 15 to 45 MPa ^1/2 , 15 to 25 MPa ^1/2 , or 30 to 45 MPa ^1/2 . Therefore, a hydrophilic polymer having a large number of polar groups may be more preferably used than a hydrophobic polymer such as polyolefins. If the solubility index is less than 15 MPa ^1/2 or more than 45 MPa ^1/2 , it may be difficult to swell with a liquid electrolyte for a typical battery.

In the porous separator, inorganic particles are filled and in contact with each other, and are bound to each other by the binder polymer, thereby forming interstitial volumes between the inorganic particles. The interstitial volumes between the inorganic particles become empty spaces and form pores.

That is, the binder polymer adheres the inorganic particles to each other so that they can maintain their bonded state, for example, the binder polymer connects and fixes the inorganic particles to each other. Also, the pores of the porous separator are pores formed when the interstitial volume between the inorganic particles becomes an empty space, and this is a space limited by inorganic particles that are substantially in contact with each other in a packed structure (closely packed or densely packed) of inorganic particles.

The binder polymer further contained in the composition may be any polymer commonly used in the art, without any restrictions. Examples include, but are not limited to, polymethyl methacrylate, polybutyl acrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate, ethylene-vinyl acetate copolymer, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, and carboxymethyl cellulose.

In this case, the binder polymer may be particulate or non-particulate. When the binder polymer is particulate, the binder polymer is not dissolved in a solvent. When the binder polymer is non-particulate, the binder polymer is dissolved in a solvent. In this case, the solvent may be an aqueous solvent or an organic solvent.

The separation membrane according to one embodiment of the present invention may further include other additives in addition to the inorganic particles and binder polymer described above as components of the porous coating layer.

Meanwhile, in a specific embodiment of the present invention, the porous coating layer may include a polymer compound represented by Chemical Formula 1.

The above-mentioned contents can be applied to the polymer compound.

When the polymer compound is further included in the porous coating layer in this way, the impregnation with the electrolyte can be further improved.

Another aspect of the present invention provides a method for producing a separator for a lithium secondary battery.

Specifically, the invention is characterized by including the steps of (S1) preparing a substrate modification composition in which a polymer compound represented by the chemical formula 1 and a surfactant are dispersed or dissolved in an aqueous solvent, and (S2) applying and drying the substrate modification composition on at least one surface of a porous polymer substrate having a large number of pores and being hydrophobic.

The details are explained below.

In step (S1), a composition for modifying a substrate is prepared in which a polymer compound represented by Chemical Formula 1 and a surfactant are dispersed or dissolved in an aqueous solvent.

In this case, the aqueous solvent may be water, and may further contain a predetermined amount of alcohol. In this case, the polymer compound and the surfactant may be dispersed in the aqueous solvent. In this case, the above-mentioned contents can be applied to the polymer compound and the surfactant.

In this case, the polymer compound and the surfactant can be dispersed in the aqueous solvent by a conventional method well known in the art, such as an ultrasonic disperser, a ball mill, a bead mill, a disperser, a mixer, etc., and a ball mill or a bead mill is particularly preferable. In this case, the processing time can vary depending on the capacity, but 1 to 20 hours is appropriate, and the particle size of the crushed polymer compound and the surfactant can be controlled by the size of the beads used in the ball mill or bead mill and the ball mill (or bead mill) time.

Next, the substrate modifying composition is applied to at least one surface of a porous polymer substrate having a large number of pores and being hydrophobic, and then dried (S2).

In the present invention, the porous polymer substrate is a porous membrane that electrically insulates the positive and negative electrodes to prevent short circuits and provide a path for lithium ions to move, and can be used without any particular restrictions as long as it can be used as a separator membrane material for ordinary electrochemical elements.

Specifically, the porous polymer substrate may be a porous polymer film substrate or a porous polymer nonwoven fabric substrate.

The porous polymer film substrate may be a porous polymer film made of a polyolefin such as polyethylene or polypropylene, and such a polyolefin porous polymer film substrate exhibits a shut-down function at a temperature of, for example, 80 to 150°C.

In this case, the polyolefin porous polymer film can be formed by using polyolefin polymers such as polyethylene (high density polyethylene, linear low density polyethylene, low density polyethylene, ultra-high molecular weight polyethylene), polypropylene, polybutylene, polypentene, etc., either alone or in combination of two or more of these.

In addition, the porous polymer film substrate may be manufactured by forming various polymers such as polyesters in addition to polyolefins into a film shape. In addition, the porous polymer film substrate may be formed in a structure in which two or more film layers are laminated, and each film layer may be formed from the above-mentioned polymers such as polyolefins and polyesters alone or from a mixture of two or more of these polymers.

In addition to the polyolefins mentioned above, the porous polymer film substrate and the porous nonwoven fabric substrate can be made of polymers such as polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyether ether ketone, polyether sulfone, polyphenylene oxide, polyphenylene sulfide, and polyethylene naphthalate, either alone or in combination.

The thickness of the porous polymer substrate is not particularly limited, but is preferably 1 to 100 μm, more preferably 5 to 50 μm. As batteries have recently been developed to have higher power and capacity, it is advantageous to use a thin film for the porous polymer substrate. The diameter of the pores in the porous polymer substrate may be 10 nm to 100 nm, 10 nm to 70 nm, 10 nm to 50 nm, or 10 nm to 35 nm, and the porosity may be 5% to 90%, preferably 20% to 80%. However, in the present invention, such numerical ranges may be easily modified according to specific embodiments or needs.

The pores of the porous polymer substrate may have various types of pore structures, and the present invention includes any one of the average pore sizes measured using a porosimeter or observed under FE-SEM that satisfies the above-mentioned conditions.

Here, in the case of commonly known uniaxially stretched dry separation membranes, the central pore size in the vertical direction (TD, transverse direction) pore size is used as the standard, rather than the pore size in the machine direction (MD, longitudinal direction) on the FE-SEM, and for other porous polymer substrates with a network structure (e.g., wet PE separation membranes), the pore size measured with a porosimeter can be used as the standard.

The method of applying the substrate modifying composition to the porous polymer substrate is not particularly limited, but it is preferable to use a slot coating or dip coating method. Slot coating is a method in which the composition supplied through a slot die is applied to the entire surface of the substrate, and the thickness of the coating layer can be adjusted according to the flow rate supplied from a metering pump. Dip coating is a method in which the substrate is immersed in a tank filled with the composition to coat it, and the thickness of the coating layer can be adjusted according to the concentration of the composition and the speed at which the substrate is pulled up from the composition tank. In order to control the coating thickness more accurately, after immersion, it may be post-weighed using a Mayer bar, etc.

By drying the porous polymer substrate coated with the substrate modifying composition in a dryer such as an oven, the fibrils in the porous polymer substrate can be modified from hydrophobic to hydrophilic. This is because the surfactant and the polymer compound have penetrated into the interior of the porous polymer substrate.

After step (S2), the method may further include a step (S3) of forming a porous coating layer by drying and applying a composition for forming a porous coating layer, which includes the solvent, inorganic particles, and a binder polymer, to at least one surface of the porous polymer substrate on which the composition for modifying the substrate has been applied and dried.

In this case, the solvent used may be an aqueous solvent or an organic solvent.

In this case, it is preferable that the solvent has a solubility index similar to that of the binder polymer to be used and a low boiling point. This is to facilitate uniform mixing and subsequent solvent removal. Non-limiting examples of usable solvents include one or a mixture of two or more compounds selected from water, acetone, tetrahydrofuran, methylene chloride, chloroform, dimethylformamide, N-methyl-2-pyrrolidone (NMP), methyl ethyl ketone, and cyclohexane.

Methods for dispersing inorganic particles can be performed by conventional methods well known in the art, such as ultrasonic dispersers, ball mills, bead mills, dispersers, mixers, etc., with ball mills or bead mills being particularly preferred. In this case, the processing time can vary depending on the capacity, but 1 to 20 hours is appropriate, and the particle size of the crushed inorganic particles can be controlled by the size of the beads used in the ball mill or bead mill and the ball mill (or bead mill) time.

The method of applying the composition for forming the porous coating layer to the porous polymer substrate is not particularly limited, but it is preferable to use a slot coating method or a dip coating method. In the slot coating method, the composition supplied through a slot die is applied to the entire surface of the substrate, and the thickness of the coating layer can be adjusted by the flow rate supplied from a metering pump. In addition, in the dip coating method, the substrate is immersed in a tank filled with the composition to coat it, and the thickness of the coating layer can be adjusted by the concentration of the composition and the speed at which the substrate is pulled up from the composition tank. In order to control the coating thickness more accurately, the substrate may be weighed using a Mayer bar or the like after immersion.

The porous polymer substrate coated with the porous coating layer-forming composition is then dried using a dryer such as an oven to form a porous coating layer on the porous polymer substrate.

In the porous coating layer, the inorganic particles and the binder polymer are filled and in contact with each other, and are bound to each other by the binder polymer, thereby forming interstitial volumes between the inorganic particles, and the interstitial volumes between the inorganic particles can become empty spaces to form pores.

That is, the binder polymer can connect and fix the inorganic particles together by adhering them to each other so that the inorganic particles can maintain their bonded state. Also, the pores of the porous coating layer are pores formed as vacant spaces in the interstitial volumes between the inorganic particles, and these can be spaces limited by inorganic particles that are substantially in contact with each other in a packed structure (closely packed or densely packed) of inorganic particles.

The drying may be performed in a drying chamber, and the conditions of the drying chamber for the application of the non-solvent are not particularly limited.

However, in the present invention, since drying is performed under humid conditions, the binder polymer may be distributed mainly on the surface of the porous coating layer. The drying step may be performed at a relative humidity of 30% or more, 35% or more, or 40% or more, and 80% or less, 75% or less, or 70% or less. For example, it may be performed in the range of 40% to 80%. In addition, the drying step may be performed at a temperature range of 20 to 70°C for 0.1 to 2 minutes.

The thickness of the porous coating layer is not particularly limited, but is preferably 1 to 10 μm, more preferably 1.5 to 8 μm, and the porosity of the porous coating layer is not particularly limited, but is preferably 35 to 65%.

Meanwhile, the composition for forming the porous coating layer may include a polymer compound including a repeating unit represented by Chemical Formula 1.

The above-mentioned contents can be applied to the polymer compound.

When a polymer compound is further included in the porous coating layer in this way, the impregnation with the electrolyte can be further improved.

An electrochemical device according to one aspect of the present invention includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, and the separator is the separator according to one embodiment of the present invention described above.

Such electrochemical devices include any device that performs an electrochemical reaction, and specifically include all kinds of primary batteries, secondary batteries, fuel cells, solar cells, and capacitors such as supercapacitor devices. In particular, the secondary batteries may be lithium secondary batteries, including lithium metal secondary batteries, lithium ion secondary batteries, lithium polymer secondary batteries, and lithium ion polymer secondary batteries.

The positive and negative electrodes to be used with the separator of the present invention are not particularly limited, and may be prepared in a form in which the electrode active material is bound to the electrode current collector by a conventional method known in the art. Among the electrode active materials, non-limiting examples of the positive electrode active material may be conventional positive electrode active materials used in the positive electrode of conventional electrochemical devices, and in particular, lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium iron oxide, or a lithium composite oxide combining these may be used. Non-limiting examples of the negative electrode active material may be conventional negative electrode active materials used in the negative electrode of conventional electrochemical devices, and in particular, lithium adsorbent materials such as lithium metal or lithium alloy, carbon, petroleum coke, activated carbon, graphite, or other carbons are preferred. Non-limiting examples of the positive electrode current collector include foils made of aluminum, nickel, or a combination thereof, and non-limiting examples of the negative electrode current collector include foils made of copper, gold, nickel, or a copper alloy, or a combination thereof.

The electrolyte used in the electrochemical element of the present invention is a salt having a structure such as A ⁺ B ^- , where A ⁺ contains an alkali metal cation such as Li ⁺ , Na ⁺ , K ⁺ or a combination thereof, and B ^- is PF ₆ ^- , BF ₄ ^- , Cl ^- , Br ^- , I ^- , ClO ₄ ^- , AsF ₆ ^- , CH ₃ CO ₂ ^- , CF ₃ SO ₃ ^- , N(CF ₃ SO ₂ ) ₂ ^- , C(CF ₂ SO ₂ ) _{3 - , or the like.} ^- or a combination thereof, dissolved or dissociated in an organic solvent such as propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethylsulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethyl methyl carbonate (EMC), γ-butyrolactone, or a mixture thereof, but is not limited thereto.

The electrolyte may be injected at an appropriate stage in the battery manufacturing process depending on the manufacturing process and desired physical properties of the final product. That is, it may be injected before the battery is assembled or at the final stage of the battery assembly.

The present invention will now be described in detail with reference to specific examples. However, the examples of the present invention may be modified into many other forms, and the scope of the present invention should not be construed as being limited to the examples described below. The examples of the present invention are provided to more completely explain the present invention to those of ordinary skill in the art.

Example 1
A composition for modifying a substrate was prepared by adding β-cyclodextrin (manufactured by TCI) and 2-[methyl[(nonafluorobutyl)sulfonyl]amino]ethyl acrylate (FC4430, manufactured by 3M) as a surfactant in a weight ratio of 99.5:0.5 to water as a solvent. The surfactant had an electrolyte permeability of 48% even after high-temperature storage. The electrolyte permeability was measured using an ultraviolet-visible spectrometer at room temperature (25° C.) after a step of preparing a composition by adding _0.5 parts by weight of the surfactant to 100 parts by weight of an electrolyte solution containing 1M LiFP6 in ethylene carbonate and ethyl methyl carbonate (3:7 volume ratio), sealing the prepared composition, and storing it in an oven at 70° C. for 6 hours.

Then, a 9 μm thick polyethylene porous film (porosity: 45%) was immersed in an impregnation bath containing the composition for modifying the substrate as a porous polymer substrate, and then dried to remove the solvent to produce a separation membrane. At this time, the sum of the β-cyclodextrin content and the surfactant content was 0.5 parts by weight based on 100 parts by weight of the polyethylene porous film. Here, the weight ratio of β-cyclodextrin to surfactant in the sum of the β-cyclodextrin content and the surfactant content (0.5 parts by weight) was maintained at 99.5:0.5, which is the weight ratio of β-cyclodextrin to surfactant contained in the original substrate material composition. The test results are shown in Tables 1 and 2.

<Comparative Example 1>
A 9 μm thick polyethylene porous film (porosity: 45%) was prepared as Comparative Example 1.

The test results are shown in Tables 1 and 2.

<Comparative Example 2>
A composition for modifying a substrate was prepared by adding 0.5 g of a surfactant (FC4430, manufactured by 3M Co.) to 99.5 ml of water as a solvent.

Then, a 9 μm thick polyethylene porous film (porosity: 45%) was immersed in an impregnation bath containing the substrate modifying composition, and then dried to remove the solvent to produce a separation membrane. At this time, the content of the surfactant was 0.5 parts by weight based on 100 parts by weight of the polyethylene porous film.

The test results are shown in Table 1.

<Comparative Example 3>
A substrate modifying composition was prepared by adding 0.5 g of β-cyclodextrin to 99.5 ml of water as a solvent.

A 9 μm thick polyethylene porous film (porosity: 45%) was then immersed in an impregnation bath containing the substrate modifying composition and then dried to remove the solvent to produce a separation membrane. At this time, the content of the β-cyclodextrin based on 100 parts by weight of the polyethylene porous film varies greatly from position to position, making it difficult to provide a representative value. The test results are shown in Table 1.

The tensile strength of the separation membrane was measured by cutting the separation membrane into a 15 mm x 100 mm sample, and then pulling the sample in both the MD and TD directions at a speed of 500 mm/min according to ASTM-D882, and measuring the strength at which the specimen broke.

The wettability profile in an electrolyte drop test was observed after 5 minutes from injecting 2 μl of an electrolyte solution of 1 M lithium hexafluorophosphate (LiPF ₆ ) dissolved in a propylene carbonate/dimethyl carbonate/ethyl methyl carbonate (PC/DMC/EMC=20:40:40 wt %) solvent onto the surface of the separator. The test results are shown in Figures 1 to 3.

Meanwhile, the water droplet contact angle was measured using a contact angle measuring device (KRUSS, Model: DSA100). Specifically, a certain amount of solution (water) was dropped onto the surface of the separation membrane, and the image was then measured to determine the angle between the substrate and the water droplet, thereby measuring the contact angle.

At this time, the heat shrinkage rate was calculated as (initial length - length after heat shrinkage treatment at 120°C for 1 hour) / (initial length) x 100.

When no surface treatment was applied, as in Comparative Example 1, the contact angle was approximately 28% larger than in Example 1, and as shown in Figure 2, it was confirmed that the wettability with the electrolyte was not improved.

As in Comparative Example 2, when cyclodextrin was not included and only a surfactant was included, it was confirmed that the electrolyte wettability was not improved, similar to Comparative Example 1.

On the other hand, as shown in Example 1 of Table 1, when a surfactant and cyclodextrin are used simultaneously, it can be confirmed that the wettability to the electrolyte is improved and the contact angle is reduced.

Claims (18)

The present invention relates to a porous polymer substrate, and a hydrophilic modified layer located inside the porous polymer substrate or on the inside and at least one surface of the porous polymer substrate, the hydrophilic modified layer including a polymer compound represented by the following Chemical Formula 1 and a surfactant:
In Chemical Formula 1, n is an integer of 6 to 8;
The content of the surfactant is 1.0 part by weight or less based on 100 parts by weight of the polymer compound. The separator for lithium secondary batteries according to claim 1, wherein the polymer compound includes α-cyclodextrin, β-cyclodextrin, γ-cyclodextrin, or two or more of these. The separator for a lithium secondary battery according to claim 1 or 2, wherein the content of the polymer compound is 1 part by weight or less based on 100 parts by weight of the porous polymer substrate. The separator for a lithium secondary battery according to any one of claims 1 to 3, wherein the surfactant is a fluorine-based surfactant, a silicone-based surfactant, a nonionic surfactant, a cationic surfactant, an anionic surfactant, or a mixture of two or more of these. As a separator for lithium secondary batteries,
The present invention relates to a porous polymer substrate, and a hydrophilic modified layer located inside the porous polymer substrate or on the inside and at least one surface of the porous polymer substrate, the hydrophilic modified layer including a polymer compound represented by the following Chemical Formula 1 and a surfactant:
The separator further includes a porous coating layer having inorganic particles and a binder polymer that fixes and connects the inorganic particles on at least one surface of the separator,
The content of the surfactant is 1.0 part by weight or less based on 100 parts by weight of the polymer compound.
In Chemical Formula 1, n is an integer of 6 to 8. The porous coating layer further includes a polymer compound represented by the following Formula 1:
6. The separator for a lithium secondary battery according to claim 5 , wherein in Formula 1, n is an integer of 6 to 8. 7. The separator for a lithium secondary battery according to claim 6 , wherein the content of the polymer compound represented by Formula 1 is 10 parts by weight or less based on 100 parts by weight of the inorganic particles. The separator for a lithium secondary battery according to claim 5 , wherein the binder polymer is particulate or non-particulate. 9. The separator for a lithium secondary battery according to claim 5 , wherein the inorganic particles are inorganic particles having a dielectric constant of 5 or more , inorganic particles having lithium ion transport ability, or a mixture of two or more thereof. The separator for a lithium secondary battery according to claim 9 , wherein the inorganic particles are hydrophilic. The separator for a lithium secondary battery according to claim 5 , wherein the inorganic particles have a silane group grafted onto the surface of the inorganic particles. The battery includes a positive electrode, a negative electrode, and a separator and an electrolyte interposed between the positive electrode and the negative electrode,
A lithium secondary battery, wherein the separator is the separator according to claim 1 . (S1) preparing a composition for modifying a substrate in which a polymer compound represented by the following formula 1 and a surfactant are dispersed or dissolved in an aqueous solvent;
(S2) applying the composition for modifying a substrate to at least one surface of a porous polymer substrate having a large number of pores and being hydrophobic, and drying the composition;
In Chemical Formula 1, n is an integer of 6 to 8;
The method for producing a separator for a lithium secondary battery , wherein the content of the surfactant is 1.0 part by weight or less based on 100 parts by weight of the polymer compound . 14. The method of claim 13 , wherein the step (S2) is a step of modifying the porous polymer substrate to be hydrophilic by permeating the surfactant and the polymer compound into the porous polymer substrate. (S1) preparing a composition for modifying a substrate in which a polymer compound represented by the following formula 1 and a surfactant are dispersed or dissolved in an aqueous solvent;
(S2) applying the composition for modifying a substrate to at least one surface of a porous polymer substrate having a large number of pores and being hydrophobic, and drying the composition;
After step (S2),
The method further includes a step (S3) of forming a porous coating layer by drying and coating a composition for forming a porous coating layer, the composition including a solvent, inorganic particles, and a binder polymer, on at least one surface of the porous polymer substrate on which the composition for modifying a substrate has been coated and dried ,
The method for producing a separator for a lithium secondary battery, wherein the content of the surfactant is 1.0 part by weight or less based on 100 parts by weight of the polymer compound .
In Chemical Formula 1, n is an integer of 6 to 8. The method for producing a separator for a lithium secondary battery according to claim 15 , wherein the solvent is an aqueous solvent or an organic solvent. The method for manufacturing a separator for a lithium secondary battery according to claim 15 or 16 , wherein the composition for forming a porous coating layer further comprises a polymer compound represented by Formula 1. The method of claim 15 , wherein the step (S3) is performed at a relative humidity of 30 to 80%.

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